Generally, heading (flowering) of rice is accelerated by short-day and delayed by long-day conditions. Among known cultivars, typically those from Kyushu and the south of Mainland Japan have strong photoperiod sensitivity whereas cultivars from the Tohoku district or Hokkaido show complete loss of such sensitivity or have extremely weak photoperiod sensitivity. Rice plants that lack the photoperiod sensitivity have a characteristic to flower after a certain length of growth period, and the heading date of the plant does not change with changes of photoperiod. Adaptation of rice plants related to cultivation locations and season drastically changes in accordance with the existence of photoperiod sensitivity in the plant. Thus, modification of photoperiod sensitivity in rice is an important aspect of breeding rice.
In conventional breeding programs, the alteration of the heading date of rice is achieved through methods involving: (1) selection of early maturing varieties or late varieties by crossing; and (2) mutagenesis by radiation and chemicals; and so on. However, such breeding programs require long periods of time to be successful, and bear other problems, such as unpredictability of the degree or direction of the mutations in the progeny.
“Photoperiod sensitivity gene” is a generic name for genes that enhance the rice photoperiod sensitivity in the field of rice genetics. The existence of several photoperiod sensitivity genes has been observed to be inherent in mutants and cultivars, and photoperiod sensitivity genes are suggested to exist on loci, for example, such as Se1 locus (chromosome 6; Yokoo and Fujimaki (1971) Japan. J. Breed. 21: 35–39), E1 locus (chromosome 7; Tsai, K. H. (1976) Jpn. J. Genet. 51: 115–128; Okumoto, Y. et al. (1992) Jpn. J. Breed. 42: 415–429), E2 locus (unknown), E3 locus (chromosome 3?; Okumoto et al. Japanese Society of Breeding, 91st lecture, Japanese Journal of Breeding 47 (Suppl. 1): 31); and so on (Yamagata et al. (1986) In Rice Genetics, International Rice Research Institute, Manilla, pp351–359).
Isolation of rice photoperiod sensitivity genes enables the introduction of such genes into arbitrary cultivars by transformation methods to modify the photoperiod sensitivity in these cultivar lines, which ultimately permits regulation of the heading date of the rice. Therefore, breeding using such genes is a particularly efficient yet simple and reliable method as compared to conventional methods.
However, the isolation of genes involved in the photoperiod sensitivity of rice has not yet been reported.